I have a new OS 91FX that I have been unable to get running properly. I have the engine mounted on a test bench. No problems with the fuel system. I set the high speed needle on the rich side ā€“ good smoke from the exhaust, and just starting to transition to 2-cycle. The engine will run for anywhere from 20 seconds to a minute, and then just stop. I know the test prop is on the small side (13x6), but I am not tweaking for RPM Iā€™m just trying to get the engine to run. I have not actually measured the head temperature, but it seems overly hot for the short run time. It is not running lean, and there are no bubbles in the fuel line to the carburetor. The engine sounds strange when I increase to full throttle - I was told it was detonating too early. I have been reading the posts here, and I started to dismiss the issue of exhaust back-pressure due to wrong/improper after-market muffler - I am using the muffler furnished with the engine. I decided to take it apart and look inside anyway. The internal cone-shaped baffle was installed backwards ā€“ cone pointing toward the exhaust port. I removed the baffle, and will test the engine next week. I suspect this will have cured my problems, but would appreciate comments as to how the baffle direction would affect operation.

Bill Vassallo

Baffle direction is largely irrelevant, but the small end of the cone should point aft to enable oil to get out more easily.

If the engine's running hot, then it's too lean, running too much propeller, or just doesn't have enough oil in the fuel. You should be using a fuel with no less than 18% oil content, and we'd strongly-suggest that you have a significant portion of the oil mix be castor oil. An all-synthetic blend would not be the best for breakin.

Here are some RPM ranges for various propellers:

13x6 12,500
13x8 10,400
13x10 9,500
14x6 10,900
14x8 9,500
14x10 8,400
15x6 10,200
15x8 8,100
16x6 8,300

Propping between 10,000 and 11,000 RPM would be a good range for most operations.

Thanks for the reply. The fuel is Omega 10%. I just looked up the formula - the total oil content is 17% (30% castor/70% synthatic). The fuel appears to be fresh - it runs fine in .46 FX. Prop is a 13x6 Zinger. RPM was just above 11,000 (rich mixture). I will change fuels, but is 1% oil really going to make a difference?

Bill Vassallo

1% in oil content shouldn't make a difference at all.